a) Field of the Invention
The present invention relates to a vibration isolator of a vibration actively reducing apparatus which interferes a vibration transmitted from a vibration source such as an engine to a supporter such as a vehicle body against a controlled vibration developed from a control vibration source intervened between the vibration source and the supporter so as to reduce the transmitted vibration. The present invention particularly relates the vibration isolator which more accurately detects a residual vibration after the interference between the vibration developed from the vibration source and the controlled vibration developed from the controlled vibration source and which drives the controlled vibration source on the basis of at least the residual vibration. The vibration isolator can accurately detect the above-described residual vibration.
b) Description of the Related Art
A Japanese Patent Application First Publication No. Heisei 7-233852 published on Sep. 5, 1995 exemplifies a previously proposed vibration isolator which detects the residual vibration.
That is to say, the above-described Japanese Patent Application First Publication discloses, so-called, an active engine mount which is interposed between the engine (vibration source) and the vehicle body so as to be enabled to generate an active supporting force.
In the above-described active engine mount, a supporting spring constituted by an elastic material made of a rubber is intervened between a casing fixed onto the engine and a center boss fixed onto a vehicle body. A fluid chamber is formed in the supporting spring in which a fluid is filled. A vibration plate which is capable of being vibrated in a direction such that a volume of the fluid chamber is varied is elastically supported by means of a plate spring. An electromagnetic actuator is fixed within a casing. Another elastic material made of the rubber is attached onto a surface of the center boss faced toward the vehicle body side. A plate-like member having a center through which a hole is provided is interposed between the other elastic material made of the rubber and the vehicle body (body frame) and a load (weight) sensor is housed in the center of the plate-like member through which the hole is provided. In details, since the load sensor is inserted between the elastic material made of the rubber and the vehicle body, a vibration force to be transmitted from the center boss to the vehicle body can be detected as a variation in the weight. Then, if the electromagnetic actuator is driven so as to reduce the output of the load sensor, i.e., the residual vibration, the vibration transmitted through the disclosed active engine mount can be reduced.
However, since, in the above-described active engine mount, the load sensor is mounted on the center boss via the elastic material made of the rubber, the output of the load sensor does not accurately correspond to the force transmitted toward the vehicle body via the engine mount due to an aging effect of the elastic material and an insufficient rigidity thereof.
In this case, it is difficult to achieve a highly accurate control of reduction in the vibration.
In addition, since, in the active engine mount disclosed in the above-described Japanese Patent Application First Publication, the load sensor is grasped between the elastic material and the vehicle body with the load sensor housed in the center of the plate-like member, a rate of the force to be transmitted to the vehicle body via the plate-like member is so high that it is difficult to detect an accurate detection of the residual vibration.
On the other hand, a Japanese Patent Application First Publication No. Heisei 8-145114 published on Jun. 4, 1996 exemplifies another active-type engine mount. Together with all forces transmitted toward the vehicle body through the disclosed active engine mount are once concentrated onto a yoke of the electromagnetic actuator and the load sensor is inserted between a flat plate portion integrated to a stud bolt fixed onto one end surface of the yoke and the end surface of the yoke and end surface of the yoke, the other end surface of the stud bolt being coupled to the vehicle body.
In this case, since the rate of the force transmitted through the load sensor becomes high, it becomes possible to detect the accurate residual vibration. Hence, in this structure, since the load sensor itself serves as a member determining a strength of the engine mount, the engine mount in its nature is disadvantageous in securing the strength of the engine mount. In other words, since the load sensor is disposed so as to enclose the stud boss, the load sensor exhibits the strength with respect to the vibration vertically transmitted through the engine mount. However, the load sensor cannot accept a part of the force with respect to the vibration in the lateral direction with respect to the engine mount.
Hence, since a burden is concentrated only to the stud bolt for the lateral vibration of the engine mount, it is necessary to sufficiently enlarge a diameter of the stud bolt so as to strengthen its strength of the stud bolt. However, there is a limit placed on the increase in the diameter of the stud bolt.
In addition, the stud bolt is utilized to grasp the load sensor. Hence, if the stud bolt is loosened, a pre-load applied to the load sensor is varied and a detection accuracy may be reduced.